The Third Generation Partnership Project (3GPP) technical specification (TS) 23.401 describes the behavior of a re-balancing function in a mobility management entity (MME). A MME is a device that hosts the following functions: non-access stratum (NAS) signaling; NAS signaling security; access server (AS) security control; inter-core network (CN) signaling for mobility between Third Generation Partnership Project (3GPP) access networks; idle mode user equipment (UE) reachability (including control and execution of paging retransmission); tracking area list management (for UEs in idle and active modes); public data network (PDN) gateway (GW) and serving GW selection; MME selection for handovers with MME change; serving general packet radio service (GPRS) support node (SGSN) selection for handovers to 3GPP access networks; roaming; authentication; bearer management functions (e.g., dedicated bearer establishment); support for earthquake and tsunami warning system (ETWS) message transmission; etc.
The MME pool architecture (e.g., defined by the 3GPP) enables radio network elements (e.g., eNodeBs) to decouple from interfacing with a single MME, and permits the radio network elements to access multiple MMEs grouped in a structure known as a “pool.” MMEs communicate with each other via S10 interfaces, regardless of whether the MMEs are part of the same pool, different pools, or operate in isolation.
If it is necessary to move UEs from a source MME to a target MME within a same pool (e.g., prior to shutting down the source MME for maintenance purposes), the 3GPP TS 23.401 standard distinguishes between UEs in an in an evolved packet system (EPS) connection management (ECM) idle mode (e.g., an ECM_IDLE mode) and UEs in an ECM connected mode (e.g., ECM_CONNECTED mode). For a UE in the ECM connected mode, 3GPP TS 23.401 recommends that the source MME initiate a S1 release procedure with a release clause (e.g., “load balancing tracking area update (TAU) required”). After initiation of the release procedure, S1 interface connections and radio resource control (RRC) protocol-based connections (e.g., associated with the source MME) are released. The UE initiates a TAU and indicates to an eNodeB (eNB) (e.g., associated with the UE) that the RRC establishment is for load balancing purposes.
However, the S1 release procedure recommended by 3GPP TS 23.401 causes a service interruption for moved UEs in the ECM connected mode. Depending on what these UEs are actually doing before the move from the source MME to the target MME, a temporary discontinuation of service would affect most applications and cause performance degradation of, for example, voice, video, and/or file transfers by the UEs.